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John W. Reed Coats English 112 30 May 1990 Public Safety Trunking Systems Thesis: The use of trunked communications systems by police departments has grown since its invention several years ago. Each year, more and more public safety agencies are changing out their conventional communications systems with trunked communications systems. They do this because the use of trunking provides the agency with a system that is in that trunking is reliable, efficient, and versatile. I. Trunking defined A. Band location B. Who invented it II. How trunking works A. Motorola 1. Smartnet 2. Smartnet II B. General Electric 1. Mastr II C. E.F. Johnson III. Why trunking is reliable A. Multiple repeater B. Fail soft systems IV. Efficiency of trunking A. Channel saving B. More people on a trunked system V. Versatility A. Phone interconnects B. Security C. County-wide systems D. It meets the needs of the user E. Computer programmability VI. Widespread use by police agencies A. Dekalb B. Gwinett John W. Reed Coats English 112 30 May 1990 Public Safety Trunking Systems Having a reliable radio communications structure in any public service agency is vital to its operation. The use of trunked communications systems by police departments has grown since its invention several years ago. Each year, more and more public safety agencies are changing out their conventional communications systems with trunked communications. They do this because the use of trunking systems provides the agency with a system that is reliable, efficient, and versatile. Land mobile radio is the fastest growing telecommunications industry (Dordick 223). In 1980, there were nearly 760,000 licensed stations. As of April 1989, there were over 1.3 million licensed stations (Popular Communications). In common land mobile radio communications, there are 4 major bands. These consist of Low Band (25-50 MHz), High Band (150-174 MHz), UHF Lo (450-470 MHz), UHF T (470-512 Mhz) and the UHF HI or 800 band (806-942 Mhz) (Hughes 3). Depending on the agency's need, different bands are selected. For a rural fire department, Low Band might be used, as Low Band provides more range and less electrical noise. For a security agency or other personnel that spends a lot of time indoors, UHF would be used, as UHF signals travel through walls unlike those of Low Band which bounce off them. Today, the lower bands, UHF, High, and Low, are extremely congested. No longer can one license a repeater, a device that automatically retransmits the signal of a mobile unit, in turn giving the mobile a greater span of communications, on those bands. The only way to go is up! Mr. Inglis adds that "the rapid growth of land mobile sources has placed continuing pressure on regulatory authorities to provide additional RF spectrum to accommodate new land mobile system users" (21.18). So why 800 MHz? 800 is the least full of all the bands. This is also the band where trunking systems exist. Trunked communications systems are licensed for five to twenty channels in the 806-942 MHz band (Hughes 7). According to Edward Loughlin, trunking is derived from the telephone system. Suppose the phone company was going to provide service to Podunk. 50 people in Podunk subscribe to the phone. Never would all 50 people be talking on the phone at the same time, and the phone company takes advantage of this. Instead of running 50 lines to each of the 50 people in Podunk, the phone company runs 12 lines. And, at any given time, 1 of those 12 lines would not be occupied. Thus, more people can be accommodated with less number of lines. The telephone company uses this same principle for routing long distance calls. Suppose a caller in Atlanta places a call to Memphis. The call goes to a long distance exchange in Atlanta, where there may be X number of lines to Memphis. The switch searches for an unused line and connects the call to that line, while the exchange in Memphis connects the line to the party being called. Trunking works all the same way. In a trunked system, five or more repeaters work together as a group (Hughes 7). According to a service manual for a General Electric Trunked System, the channels are "uniformly spaced 25 KHz apart with the repeater transmitting 45 MHz higher than the mobile." Suppose a trunked system has 20 repeaters. All of the mobiles and base units are programmed to operate on any of the twenty channels. Similar to cellular phones, the programming and switching of frequencies is accomplished with the aid of computers. Unlike cellular phones, a trunked system does not switch to another channel in the middle of a conversation. There are three major types of trunking systems: Motorola, General Electric, and E. F. Johnson (Hughes 7). More or less, they all work the same way. When a mobile user presses the microphone button, the radio automatically tunes to an empty channel. At the repeater site a computer then tunes all the other radios in the system to the same channel. If all the channels are busy, a flashing "busy" light will be activated on the mobile radio (Kay 10). According to Edward Loughlin, at any given time one channel should be available. There are distinct differences, however. In the General Electric system, each radio constantly scans all available channels. When the microphone button is pressed, the scanning stops on a vacant channel and the radio transmits "a fleet- specific code" (Hughes 7). All radios with the same code switch to that particular channel. When the microphone button is released, the repeater holds a few seconds to allow a unit to reply before scanning starts again. Each repeater in the system stands alone, and its failure would not be catastrophic to the rest of the system. The E.F. Johnson system is quite different. Each group of radios is assigned a channel in the system and it is designated the "home" channel. All the repeaters are linked together via phone or other high speed transmission lines. If the home channel is busy, a radio originating the call will switch to an unused channel, and simultaneously, send a burst of data that will connect the other units to the new channel. Each time the microphone button is released, the repeater deactivates and returns all units to the original home channel. Of course, in the Johnson system, there is a possibility of the system becoming full. The Motorola system, in my opinion, is the best of all trunking systems. On a twenty channel trunking system, one of the channels is designated the data channel. All radios in the system are tuned to the data channel. When the microphone button is pressed, a burst of information is sent to the repeater site controller, a computer. The controller then switches all radios in that particular person's group to an empty channel. If all channels are busy, the radio group goes on a computerized waiting list and is literally called back when a channel becomes free. If the controlling computer happens to fail, each radio in a group, and there can be many groups to a system, reverts to a predetermined base channel. The system then functions as a conventional repeater system. Trunking is far more reliable than a conventional repeater system. In a conventional system, a service may have one or more separate radio channels. Sometimes, in a shared conventional system ( a system with more than one user ), a user would have to wait his turn. If all the stations on the system try to talk at once, the result is more madness! (Hughes 7) The trunked system, however, allows for all users in the system. When a group in the trunked system is allocated a channel by the system, all other users on the system are locked out of that particular channel. This prevents eavesdropping by the other users and also prevents interference from the other groups, like being covered up or "stepped on." A trunking system has an incredibly high level of reliability. A trunked user would, normally, not be aware of a failure of any of the repeaters in the system. If the system controller recognizes a failure on one of the channels, it locks that channel out of the system. The system then redistributes the remaining channels. Should the control repeater fail, the controller assigns one of the other repeaters as the control repeater. In a conventional system, if the repeater malfunctions and the users do not have a backup repeater, the users are cut off. This could be disastrous in an Ambulance or Police communications system. In a trunked system, likewise, trouble would occur if the system computer malfunctioned. In a public safety trunking system, there is usually a backup computer, or a system that "distributes the control logic in such a way that failure does not disable the system" (Hughes 8). In a trunking system, if the system computer and its backup fails, the system goes into Fail Soft mode. Each group on the system is preassigned to one of the channels which then functions as a conventional repeater. The Motorola system takes advantage of this wholeheartedly. Trunking is, by far, the most efficient means of land mobile communication. Theoretically, if there are five channels, each channel is not busy one fifth of the time. There should always be one unused channel. Trunking makes use of the channels by distributing the load resulting in efficiency (Hughes 7). A non trunked single channel repeater could accommodate fourteen mobile units with fourteen units per channel. On a five channel trunking system with one of the channels as a data or control channel, two hundred sixty mobiles could be accommodated with fifty-two units per channel. In a twenty channel trunking system with one channel as the data channel, one thousand nine hundred mobile units could be accommodated at ninety-five units per channel (Inglis 21.22). Trunked systems are more versatile than a conventional system. True, a conventional system can be fitted with a phone interconnect, but the trunking phone interconnect is full duplex, it receives and transmits at the same time like a normal phone (Fowler). Plus, "the addition of computer like functions and control signaling techniques have greatly expanded the capacity of two-way radio systems" (Dordick 223). Unlike a conventional system, a trunked system has the ability to reprogram all of the radios in the field at the same time and over the air. For example, in a police trunking system there is a group for traffic enforcement and a separate group for homicide. There is a high speed chase which results in a murder. The base can reprogram the detectives' radios to operate on the traffic enforcement group while the detectives' are enroute to the scene. For a conventional system, the detectives would have to have the traffic channel preprogrammed on a separate channel or they would have to take the radios back to the base to be reprogrammed by a computer. Plus, according to Ben Ho, trunking equipment can be interfaced with data equipment (55). For example, a police car could have a terminal in the vehicle to run license checks without having to call the base. Ho also says that the use of trunking makes expandability cost effective in the long run (55). Day by day, more and more agencies are deciding to move from their old radio system to a trunked radio system. Years ago, Manual Maloof, Executive Officer for DeKalb County, opted to put the entire county on a trunked system. All county services are now on their public safety trunking system. This provides the police the ability to talk to the sheriff which can then talk to the fire department, all on the same system. DeKalb has its system placed on top of Stone Mountain which provides the county superior radio coverage. Cobb County Water Department is also on a trunked system. It will not be too long before the entire town goes trunking. Gwinnett County also has a set of trunking channels licensed to them. It will only be a matter of time before they go to trunking. Fulton County is on trunking, as is Clayton County. Police cars were equipped with radios as early as 1926 (Dordick 223). Little did they know that radio communication would traverse so far. Inglis remarks that it is obvious that trunking is beneficial to the preservation of radio spectra. Figures prove that a 20 channel system (19 with one data channel) can provide seven times the number of a single channel repeater system with the same if not increased quality of service. (21.22) With today's congested band space, it is vital to come up with new efficient means of communications. At the same time, it is vital for public service agencies to have a competent communications system. Without, lives could be lost, and the safety of the user could be impaired. Works Cited Dordick, Herbert S., Understanding Modern Telecommunications. New York: McGraw-Hill, 1986. Fowler, Ralph W. Personal Interview. 30 April 1990. General Electric Company. Training Manual for G.E. Marc V Trunking System. Lynchburg, VA: General Elecrtic Company, 1983. Ho, Ben. "Centralized Dispatching and Tracking Viz Mobile and Packet Radio." Telecommunications August 1986 : 55. Hughes, Gene, ed. Police Call. Los Angeles: Hollins Radio Data 1988. Inglis, Andrew F, ed. Electronic Communications Handbook. New York: McGraw-Hill, 1988. Kay, Bob. "Trunk Busting Basics for Scanner Listeners." Monitoring Times Sept 1988: 10-11. Loughlin, Edward. Personal Interview. 8 May 1990. "More Frequencies Proposed for Trunked Technology in the Private Land Mobile Radio Services." Popular Communications April 1989: 50. Watts, Frank. Personal Interview 21 May 1990.